Such service units are known to be employed in connection with open-end spinning machines as disclosed, for example, in German Patent Publications DE 44 43 818 A1, DE 38 01 964 A1, DE 42 44 081 A1 or DE 44 04 538 A1.
The service units described in these patents service the numerous work stations of the open-end spinning machines, i.e. they act autonomously in case a need for a service operation or action occurs at one of the work stations. Such a need arises, for example, if a yarn break has occurred at one of the work stations, or if a cheese at one of the work stations has attained its prescribed full diameter and must be exchanged for an empty tube.
In such case, the service unit moves to an operative position at the respective work station and, in case of an ordinary yarn break, i.e., a yarn break occurring during the course of winding the cheese to its prescribed diameter, the service unit actuates a yarn searching nozzle to locate and aspirate the torn yarn end lying on or extending from the circumferential surface of the cheese. The yarn end is prepared for continuation of the spinning process and, after the spinning rotor has been cleaned, the yarn end is reinserted into the spinning box of the work station.
In the case of the service units known from German Patent Publications DE 44 43 818 A1 or DE 38 01 964 A1, the prepared yarn end is attached to a fiber ring formed from individual fibers, which rotates along with the spinning rotor, while with the service units in accordance with German Patent Publications DE 42 44 081 A1 or DE 44 04 538 A1 the delivery of fibers to the spinning rotor from the sliver opening device is first completely rerouted and is only re-fed to the fiber collecting surface of the spinning rotor after conditions are appropriate for resuming the spinning operation.
Since a pieced portion of a yarn should differ as little as possible from the remaining extent of the yarn as normally spun by the spinning rotor in regard to the appearance as well as the strength of the pieced portion of yarn, various setting parameters, for example the number of revolutions at the spinning start, the feed-in of the fiber end and the yarn retraction, should be exactly maintained. It is therefore known to check after each piecing operation whether the yarn profile in the area of the piecing device falls within preset acceptable values. Thus, the values determined during the check of the piecing operation can be processed, as described in German Patent Publication DE 196 49 314 A1, in a control device of the service unit in such a way that subsequent yarn piecings are optimized. Here, the optimization of the yarn piecing is performed by means of a defined small change of one or several of the setting parameters which are responsible for the condition of the yarn piecing.
This service process becomes somewhat more difficult when an empty tube must be exchanged for a full finished cheese and a fresh spinning process must be started thereafter. In this case, following the ejection of the finished cheese and the insertion of an empty tube, an auxiliary yarn is initially provided by an auxiliary yarn feeding device arranged in the service unit.
As described, for example, in German Patent Publication DE 44 43 818 A1, the auxiliary yarn, which extends between the mouth of the feed tube of the auxiliary yarn feeding device and the yarn search nozzle, is used for piecing by the yarn piecing devices of the service unit. Thus, the auxiliary yarn is first positioned in front of a yarn catching plate and is transferred to the yarn feeding device by a yarn draw-in device. Subsequently the auxiliary yarn, which is gripped in feeding tongs of the yarn feeding device, is cut by means of a yarn cutting device, and the severed yarn end is removed by the yarn search nozzle via suction. The end of the auxiliary yarn end gripped in the feeding tongs of the yarn feeding device is finally prepared in a customary manner, transferred by the forward pivoting yarn feeding device to the spinning box of the appropriate work station of the open-end spinning machine and is threaded into a small yarn withdrawal tube of the spinning box. Shortly before the auxiliary yarn is placed into the rotating spinning rotor against the ring of individual fibers therein, the auxiliary yarn feeding device is switched to suction air and the auxiliary yarn is cut by a yarn cutting device.
Thereafter the auxiliary yarn and the freshly spun yarn are drawn off by means of the yarn withdrawal device of the service unit and aspirated into the feed tube of the auxiliary yarn feeding device. Then a length of yarn including the auxiliary yarn and the reattached piecing of spun yarn are cut off by a yarn cutting device arranged in the area of the feed tube, and the freshly spun yarn is placed against an empty winding tube maintained between the tube plates of the bobbin creel of the respective work station.
Since the auxiliary yarn used by the above described service units often is not identical with the yarn to be subsequently produced, for example in case of the start of a batch, problems can sometimes arise in the course of the piecing process.